Stencil

Stencil Matters

Joe Clarke has spent the past 47 years in the lab and in the engineering department, in pre-press and on-press, as an R&D / technical researcher and as a manager of screen print production. Clarke has held executive positions as President of M&R Printing Equipment and as Vice-President at Wilflex [Poly One]. He has been granted a growing number of print-related patents, including one for High-Shear printing with Smilin'Jack - he is a member of the ASDPT, is an Associate Editor for NBM and an SGIA Fellow.

Clarke has presented hundreds of technical papers, written a couple books and published over 600 technical / management articles for which he has been awarded five Swormstedts; the international standard for excellence in technical writing.

Currently Joe Clarke is the President of CPR, a Chicago-based corporation which manufactures Synergy Inks including NexGen; environmentally & financially responsible T-Shirt inks. For more information on CPR, visit http://www.cprknowsjack.com/.

A quality stencil can’t guarantee a top-quality print any more than any other single element of the process can do so. But an inferior stencil can absolutely, unequivocally ensure a less-than-top-quality image. It also often causes slow print speeds with lots of stops during the press run. So, let’s take a look at building a good stencil, which starts with defining the terms deposit, mesh, stencil and squeegee blades.

Deposit, mesh, stencil & squeegees

Deposit is mostly used as a generic term, but on closer inspection, we find there is more than meets the eye. Deposit implies two facets of coverage; wet-film (ink) thickness and ink volume. Let’s say you want two mils (0.002") of ink. In this case, you’d select, say, a 100/50 mesh which permits slightly less than 2 mils of ink thickness, apply a stencil and select a pair of squeegees that deliver the right volume.

Here, understand the principle that the mesh demands, but the stencil contains and the blades deliver. So the first fundamental “stencil problem” is due to poor mesh selection. We erroneously select a mesh count that allows for detail, then opt for a thread count that keeps the wrong mesh from ripping too soon. No matter how smooth this high count/thick thread mesh may seem, it is nothing but peaks and valleys—and we’re going to try to put a smooth coating on it? For the sake of the image and the stencil, select a high-volume screen mesh.

What’s a stencil to do?

The best way to make provisions for a quality stencil is to give it a smooth substrate surface. Select a mesh with the highest percentage open area (OPA%) and orient the gloss side of this mesh to be against the garment when on press. Minimize both the squeegee pressure (downward force) and the contact area (that between the blade and mesh). Run the fastest practical flood and stroke speeds, which imposes the minimum force on the mesh, and breathe life into the stencil.

Stencils are highly application-sensitive. This means nearly all of them will perform as advertised if and when they are used properly. But it is inadvisable to use the stencil to make up for a high volume mesh, tacky inks with poor flow properties or buckled blades.

To make predictable and consistent prints, we want the minimum emulsion over mesh (EOM) so we don’t detract from the shearing force. Use the proper mesh (not the stencil) to determine the desired wet-film thickness, and minimize the EOM to keep the ink volume sufficiently low. Edge acuity is defined by the stencil and supported by the squeegees, the RzS1 (squeegee side flatness) must be as smooth as possible.

In the above example, we put a round black dot encircled with white over top of the ideal magenta dot. This makes the seepage of magenta ink very conspicuous. It is due to poor stencil flatness on both sides, dot stacking and a buckled blade. This image will have a “bluish-red” haze.

A note on thickness

Articles, presentations and conversations on the subject of stencils all seem to indicate emulsion over mesh (EOM) as the priority—this should rarely be the case. Once the stencil is “flat enough” on the squeegee side, a thicker stencil will actually alter the ink deposit for image elements that are less than 0.080" (2mm).

The stencil only affects the edge of the image and the volume delivered at the perimeter of the image elements. With larger areas, an overly-thick stencil will exaggerate the edge and increase the contrast between the edge and the middle of a print. Finally, stencils don’t add to the “thinning effect” of the ink deposit like the blades or the mesh. Thick stencils require a delayed viscosity recovery—not so bad when we want penetration, but a thick stencil is deadly when we want matte-down or coverage.

Unfortunately, there are SFX inks and additives such as high-density where the blades are incapable of filling the cells of the mesh sufficiently. In such cases, do as you will with the stencil, but plan to make lots of passes, each with its own flash. Otherwise remember: EOM is a result of creating the proper thickness on the squeegee side of the mesh (RzS1)—nothing more.

Process, Index or Simulated?

Every substrate in every industry—offset, flexo, gravure, digital and even screen-printed graphics—use four-color process… except the T-shirt printers! If we ask the garment printer why, they’ll tell us “because index ‘pops’ more,” or “because we need opacity,” or the ethereal “four-color process doesn’t work on T-shirts.” Truth be known, it’s because we cheat ourselves, the dots and the color when it comes to the stencil. We think “the trick” is in the separations and the mesh. When we want to print top-quality simulated or true process on T-Shirts, we bite the bullet and use capillary film.

Capillary film makes uniquely consistent RzS1 and EOM. However, if we look at a cross-section of mesh, it looks like an hourglass—a funnel on top, narrow in the middle connected to another funnel on the bottom. This shape can lead to “dot stacking” which, of course, results in muddy four-color process images. But, the capillary film fixes the substrate side of the screen and applying a direct emulsion on the squeegee side of the screen turns two funnels into one tunnel to improve dot accuracy and to balance the tonal range.

Water-based systems

When printing with a water-based ink system, also known as “the arch-enemy of nearly all stencil systems,” the best choice is an emulsion developed specifically for it. Stencils tend to break down easier with water-based inks.

To build a more durable stencil, we must know the value of face coats and apply interior (squeegee side) coatings on the screen, make certain to dry it thoroughly and expose it completely. If we want to be able to expose the interior coating and not have it wash down the drain, we must have a coating which is very flat/has a low thickness on the garment side. To dry it completely (which rarely occurs outside of Arizona), we need to give it time and more time to accommodate the interior coating. Lastly, we rarely see a screen which is over-exposed but we can’t expose the water out of a stencil.

The key to stencil durability is to flood, don’t fill, and leave a good cover of ink on the mesh; use a hinged printing blade with a sharp edge; minimize the angle and print at top speed. This will keep the ink from accumulating and eventually drying in the mesh/stencil. If so, we won’t need to wash the screen as often. So, automatically, the stencil lasts longer.

Pay me now…

I have taken the old axiom; “pay me now or pay me later” and outfitted it to reflect the stencil scenario: “Pay me (a teeny-tiny bit) now or pay me a whole lot more later, on-press”. To cover for our expenses, our presses need to generate $2 (manual) to $4 (automatic) per minute. Consider that next time the temptation arises to use a cheap emulsion, lose sight of capillary quality, skip a face coat of emulsion, expose before the screens are dry or minimize the exposure.